The present invention relates to rocket engine nozzles, and more particularly to nozzles that will be subjected to jet-separation phenomenon that gives rise to high levels of lateral force.
To obtain high specific impulse at altitude, rocket engines are fitted with nozzles having a large section ratio. For a given engine, as it rises through the atmosphere, ambient pressure decreases, dropping from atmospheric pressure at sea level to a low pressure that is a function of altitude. Nozzles are generally optimized as a function of the overall performance of the launcher, which leads to using over extended nozzles for which thrust reaches a maximum at a so-called “critical” altitude. Consequently, at the atmospheric pressure to be found at sea level, the extent to which gas can expand in the nozzle is limited by a phenomenon whereby the jet separates from the wall of the diverging portion.
This phenomenon exists throughout the stage of flight that extends from lift-off up to the critical altitude, which may be situated for example at about ten kilometers from the ground, and at which thrust reaches a maximum because the static pressure of the gas in the outlet section of the nozzle is then equal to ambient pressure, which is relatively low. Throughout this stage of flight, the static pressure of the gas in the outlet section of the nozzle is well below ambient pressure, thereby leading to the phenomenon of the jet separating within the nozzle, which phenomenon disappears at the critical altitude. The jet-separation phenomenon also occurs on the ground while the engine is being stopped.
Jet separation is particularly large during stages of starting the engine on the ground, or stopping it, where applicable. In particular, there is a risk of the lateral forces generated by jet separation breaking the attachment between the engine and the launcher, which can have consequences that are harmful or even catastrophic for the launcher and the launch base (destruction of the launch pad, etc.).
To limit the forces due to jet separation, various types of solution have already been proposed. One of these solutions consists in taking of the lateral forces acting on the diverging portion by means of mechanical drive assemblies that are distributed around the nozzle. With such assemblies, in the event of the jet separating, control actuators transmit a bearing reaction to the diverging portion in the opposite direction to the lateral force generated on the diverging portion. The energy delivered to the diverging portion by jet separation is transformed into kinetic energy of the engine tilting. If jet separation persists in amplitude and direction, the maximum stroke of the drive assembly in question is reached and it comes into abutment. On coming into abutment, elastic deformation of the drive assembly enables the kinetic energy of the engine to be dissipated as potential energy of deformation. Nevertheless, if the level of energy to be dissipated is too great, the forces on coming into abutment exceed the limits of the parts concerned, thereby causing them to break, and possibly destroying the launcher on the launch pad.